Non-aqueous electrolyte secondary batteries

Abstract

This non-aqueous electrolyte secondary battery comprises: a non-aqueous electrolyte which contains a sulfonylimide compound and a chain carbonate-based solvent and / or a saturated cyclic carbonate-based solvent, and contains at least one selected from the group consisting of carbonic acid components, unsaturated cyclic carbonate-based compounds, compounds represented by general formula (4) of MPOcFd, and phosphorus atom-containing compounds represented by general formula (5) of [-P(=O)(OR1)O-]n; a negative electrode which contains first graphite having a D / G ratio of 0.7 or a G band half-value width of greater than 28 cm-1 and 0-10 mass% of second graphite, of which the D / G ratio or half-value width is equal to or less than the above value, with respect to 100 mass% of the total amount of the first graphite and the second graphite; and a positive electrode.

Description

[Technical Field]

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery. [Background technology]

[0002] In order to improve the battery performance of secondary batteries such as lithium ion secondary batteries, various non-aqueous electrolyte solutions and materials for use in secondary batteries have been investigated. Through previous investigations, the present inventors have found that a non-aqueous electrolyte solution containing a sulfonylimide compound such as lithium bis(fluorosulfonyl)imide as an electrolyte salt improves the battery performance of lithium ion secondary batteries, such as high-temperature durability and charge / discharge cycle performance.

[0003] As a result of further investigation, the present inventors have found that batteries using a non-aqueous electrolyte containing a sulfonylimide compound exhibit greater self-discharge from a fully charged state during storage than batteries using a non-aqueous electrolyte containing only a lithium compound other than a sulfonylimide compound (e.g., LiPF6, LiBF4, etc.) as the electrolyte salt, and that there is room for improvement in the storage characteristics of batteries. They have therefore proposed various techniques for improving this (e.g., Patent Document 1).

[0004] The present inventors have also proposed a non-aqueous electrolyte secondary battery, which is a secondary battery equipped with a non-aqueous electrolyte, and which comprises a non-aqueous electrolyte containing a sulfonylimide compound and a sulfone compound, and a negative electrode containing a specific carbon material (Patent Document 2).

[0005] As non-aqueous electrolytes for secondary batteries, for example, Patent Documents 3 to 6 propose non-aqueous electrolytes containing additives such as trimethylsilyl polyphosphate. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2022 / 065198 [Patent Document 2] Patent No. 6646522 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-91785 [Patent Document 4] Japanese Patent Application Publication No. 2016-192401 [Patent Document 5] Korean Patent Publication No. 2017-0000903 [Patent Document 6] International Publication No. 2016 / 209840 Summary of the Invention [Problem to be solved by the invention]

[0007] Patent Document 1 describes that adding a fluorophosphate compound such as vinylene carbonate (VC) or lithium difluorophosphate (LiPO2F2) or dissolving a carbonate component such as CO2 to a non-aqueous electrolyte solution containing a sulfonylimide compound can suppress self-discharge of a battery using the same. However, the crystallinity of graphite used as the negative electrode active material is not studied or described.

[0008] Patent Document 2 does not consider or describe the self-discharge of a battery using a non-aqueous electrolyte containing a sulfonylimide compound. Incidentally, the non-aqueous electrolyte secondary battery of Patent Document 2 exhibits a Raman spectrum intensity ratio R (1350 cm) when excited with an argon laser having a wavelength of 532 nm. -1 Peak intensity / 1580cm -1 The negative electrode is made of a carbon material having a peak intensity (R) of 0.1≦R≦0.5, but such a carbon material (graphite) has high crystallinity and is relatively expensive.

[0009] Patent Documents 3 to 6 do not provide detailed descriptions of the non-aqueous electrolyte solution containing a sulfonylimide compound and the negative electrode as components of the secondary battery, and do not discuss the crystallinity of graphite used as the negative electrode active material.

[0010] The present disclosure has been made in view of the above points, and an object of the present disclosure is to suppress self-discharge (improve storage characteristics) in a nonaqueous electrolyte secondary battery including a nonaqueous electrolyte containing a sulfonylimide compound, by combining an additive used in the nonaqueous electrolyte with a negative electrode containing graphite, which has low crystallinity and is relatively inexpensive, as a negative electrode active material. [Means for solving the problem]

[0011] It was found that in non-aqueous electrolyte secondary batteries equipped with a non-aqueous electrolyte containing a sulfonylimide compound, a first battery using a negative electrode containing graphite with low crystallinity (e.g., natural graphite, "first graphite" described below) exhibits greater self-discharge during battery storage than a second battery using a negative electrode containing graphite with high crystallinity (e.g., artificial graphite, "second graphite" described below). Based on this finding, the inventors of the present application conducted further studies and found that a third battery using a negative electrode containing graphite with low crystallinity in combination with a non-aqueous electrolyte containing a specific additive exhibits less self-discharge than the second battery.

[0012] To achieve the above object, the disclosed technology aims to suppress self-discharge in a non-aqueous electrolyte secondary battery containing a sulfonylimide compound by using low-crystalline graphite in combination with a specific additive.

[0013] The nonaqueous electrolyte secondary battery of the present disclosure contains an electrolyte salt having the general formula (1): LiN(RSO2)(FSO2) (R represents a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or a fluoroalkyl group having 1 to 6 carbon atoms.) (1) and at least one carbonate solvent selected from the group consisting of chain carbonate solvents and saturated cyclic carbonate solvents as an electrolyte solvent, and carbon dioxide (CO2), carbon monoxide (CO), bicarbonate ion (HCO3 - ) and carbonate ions (CO3 2-), and / or an unsaturated cyclic carbonate compound, a compound represented by the general formula (4): MPO c F d (M: alkali metal element, c: 1≦c≦3, d: 1≦d≦3) (4) and compounds represented by general formula (5): [-P(=O)(OR 1 )O-] n ···(5) (In formula (5), R 1 represents an alkyl group having 1 to 6 carbon atoms (which may have a substituent), a fluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), an aryl group (which may have a substituent), a silyl group (which may have a substituent), an alkali metal atom, an onium salt or a hydrogen atom, and n represents 2 or more. a non-aqueous electrolyte solution containing at least one phosphorus atom-containing compound selected from the group consisting of: a negative electrode containing, as a negative electrode active material, a first graphite having a peak area ratio (D / G ratio) of a D band to a G band analyzed by Raman spectroscopy of greater than 0.7, and a second graphite having a D / G ratio of 0.7 or less in an amount of 0% by mass or more and 10% by mass or less relative to 100% by mass of the total amount of the first graphite and the second graphite; The battery is characterized by comprising a positive electrode.

[0014] The nonaqueous electrolyte secondary battery of the present disclosure comprises the nonaqueous electrolyte and The negative electrode active material has a G-band half-width of 28 cm as analyzed by Raman spectroscopy. -1 It contains primary graphite that is larger than 28 cm and has a G-band half-width of 28 cm. -1 a negative electrode containing a second graphite having the following content in an amount of 0% by mass or more and 10% by mass or less, relative to 100% by mass of the total amount of the first graphite and the second graphite; The battery is characterized by comprising a positive electrode.

[0015] In the nonaqueous electrolyte secondary battery of the present disclosure, the sulfonylimide compound represented by the general formula (1) may contain LiN(FSO2)2. The unsaturated cyclic carbonate compound may contain vinylene carbonate. The compound represented by the general formula (4) may contain at least one selected from the group consisting of Li2PO3F and LiPO2F2. The compound represented by the general formula (5) may contain at least one selected from the group consisting of trimethylsilyl polyphosphate, ethyl polyphosphate, (triisopropylsilyl) polyphosphate, and [(tert-butyl)dimethylsilyl] polyphosphate. The additive includes a phosphorus atom-containing compound represented by the general formula (5), and the compound represented by the general formula (5) may contain at least one selected from the group consisting of trimethylsilyl polyphosphate, ethyl polyphosphate, (triisopropylsilyl) polyphosphate, and [(tert-butyl)dimethylsilyl] polyphosphate. The electrolyte salt is represented by the general formula (2): LiPF a (C m F 2m+1 ) 6-a (a:0≦a≦6, m:1≦m≦4)···(2) A compound represented by general formula (3): LiBF b (C n F 2n+1 ) 4-b (b:0≦b≦4, n:1≦n≦4)···(3) and LiAsF6. [Effects of the Invention]

[0016] According to the present disclosure, in a nonaqueous electrolyte secondary battery including a nonaqueous electrolyte containing a sulfonylimide compound, self-discharge can be suppressed (storage characteristics can be improved) by combining an additive used in the nonaqueous electrolyte with a negative electrode containing graphite, which has low crystallinity and is relatively inexpensive, as a negative electrode active material. [Brief explanation of the drawings]

[0017] [Figure 1]Figure 1 shows the D / G chart (Raman spectrum) of the graphite "MAGE" used in the manufacturing example. [Figure 2] Figure 2 is the D / G chart for the graphite "SFG15" used in the manufacturing example. [Figure 3] Figure 3 is the D / G chart for the graphite "SLP50" used in the manufacturing example. [Figure 4] Figure 4 is the D / G chart for the graphite "O-MAC" used in the manufacturing example. [Figure 5] Figure 5 is the D / G chart for the graphite "SMG" used in the manufacturing example. [Figure 6] FIG. 6 shows the 31P-NMR spectrum of the reagent trimethylsilyl polyphosphate (PPSE-1) used in Example 4 series. [Figure 7] FIG. 7 shows the 31P-NMR spectrum of polytrimethylsilyl phosphate (PPSE-2) synthesized in Example 4 series. [Figure 8] FIG. 8 shows the 31P-NMR spectrum of polytrimethylsilyl phosphate (PPSE-3) synthesized in Example 4 series. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present embodiment will be described in detail below with reference to the drawings. The following description of the preferred embodiment is merely exemplary in nature and is not intended to limit the present invention, its applications, or its uses.

[0019] <Nonaqueous electrolyte secondary battery> The nonaqueous electrolyte secondary battery according to this embodiment is a secondary battery including a nonaqueous electrolyte, a positive electrode, and a negative electrode.

[0020] [Nonaqueous electrolyte] The non-aqueous electrolyte contains an electrolyte salt, an electrolyte solvent, and an additive.

[0021] (electrolyte salt) The electrolyte salt has the general formula (1): [C1] LiN(RSO2)(FSO2)···(1) (hereinafter referred to as "sulfonylimide compound (1)", a fluorine-containing sulfonylimide salt) represented by the following formula: That is, the nonaqueous electrolyte secondary battery according to this embodiment includes a nonaqueous electrolyte containing sulfonylimide compound (1) as an essential component as an electrolyte salt as one of its constituent materials.

[0022] In the general formula (1), R represents a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or a fluoroalkyl group having 1 to 6 carbon atoms.

[0023] Examples of the alkyl group having 1 to 6 carbon atoms include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, a pentyl group, and a hexyl group. Among the alkyl groups having 1 to 6 carbon atoms, a linear or branched alkyl group having 1 to 6 carbon atoms is preferred, and a linear alkyl group having 1 to 6 carbon atoms is more preferred.

[0024] Examples of the fluoroalkyl group having 1 to 6 carbon atoms include alkyl groups having 1 to 6 carbon atoms in which some or all of the hydrogen atoms have been substituted with fluorine atoms. Examples of the fluoroalkyl group having 1 to 6 carbon atoms include a fluoromethyl group, a difluoromethyl group, a trifluoromethyl group, a fluoroethyl group, a difluoroethyl group, a trifluoroethyl group, and a pentafluoroethyl group. In particular, the fluoroalkyl group may be a perfluoroalkyl group.

[0025] The substituent R is preferably a fluorine atom or a perfluoroalkyl group (for example, a perfluoroalkyl group having 1 to 6 carbon atoms, such as a trifluoromethyl group, a pentafluoroethyl group, or a heptafluoropropyl group), more preferably a fluorine atom, a trifluoromethyl group, or a pentafluoroethyl group, still more preferably a fluorine atom or a trifluoromethyl group, and still more preferably a fluorine atom.

[0026] Specific examples of the sulfonylimide compound (1) include lithium bis(fluorosulfonyl)imide (LiN(FSO2)2, LiFSI), lithium (fluorosulfonyl)(methylsulfonyl)imide, lithium (fluorosulfonyl)(ethylsulfonyl)imide, lithium (fluorosulfonyl)(trifluoromethylsulfonyl)imide, lithium (fluorosulfonyl)(pentafluoroethylsulfonyl)imide, and lithium (fluorosulfonyl)(heptafluoropropylsulfonyl)imide. The sulfonylimide compounds may be used alone or in combination of two or more. The sulfonylimide compound (1) may be a commercially available product or may be synthesized by a conventional method.

[0027] Among the sulfonylimide compounds (1), from the viewpoint of improving battery performance, LiN(FSO2)2, lithium (fluorosulfonyl) (trifluoromethylsulfonyl) imide, and lithium (fluorosulfonyl) (pentafluoroethylsulfonyl) imide are preferred, with LiN(FSO2)2 being more preferred. In other words, among nonaqueous electrolytes, those containing LiN(FSO2)2 as the sulfonylimide compound (1) are preferred.

[0028] The concentration (content, total content when two or more types are used) of the sulfonylimide compound (1) in the nonaqueous electrolyte is preferably 0.01 mol / L or more, more preferably 0.05 mol / L or more, even more preferably 0.1 mol / L or more, still more preferably 0.2 mol / L or more, and even more preferably 0.5 mol / L or more from the viewpoint of improving battery performance. Moreover, from the viewpoint of suppressing a decrease in battery performance due to an increase in the viscosity of the electrolyte and self-discharge of the battery, the concentration is preferably 5 mol / L or less, more preferably 3 mol / L or less, and even more preferably 2 mol / L or less.

[0029] From the viewpoint of improving battery performance, the content of the sulfonylimide compound (1) in the non-aqueous electrolyte solution is preferably 10 mol % or more, more preferably 20 mol % or more, even more preferably 30 mol % or more, and even more preferably 50 mol % or more, based on a total of 100 mol % of the electrolyte salts contained in the non-aqueous electrolyte solution.

[0030] The content of sulfonylimide compound (1) in the non-aqueous electrolyte is preferably 1% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more, based on the total amount of the components contained in the non-aqueous electrolyte (100% by mass), from the viewpoint of improving battery performance. Furthermore, the concentration is preferably 70% by mass or less, more preferably 50% by mass or less, even more preferably 30% by mass or less, and even more preferably 20% by mass or less, based on the total amount of the components contained in the non-aqueous electrolyte (100% by mass), from the viewpoint of suppressing a decrease in battery performance due to an increase in the viscosity of the electrolyte.

[0031] The electrolyte salt (lithium salt) may contain the sulfonylimide compound (1), but may also contain other electrolyte salts (electrolyte salts other than the sulfonylimide compound (1)). Examples of other electrolytes include imide salts and non-imide salts.

[0032] Examples of imide salts include fluorine-containing sulfonylimide salts other than sulfonylimide compound (1) (hereinafter referred to as "other sulfonylimide compounds"). Examples of other sulfonylimide compounds include non-lithium salts of the fluorine-containing sulfonylimides listed as sulfonylimide compound (1) (for example, salts in which the lithium (ion) in sulfonylimide compound (1) is substituted with a cation other than lithium ion). Examples of salts in which a cation other than lithium ion is substituted include alkali metal salts such as sodium salt, potassium salt, rubidium salt, and cesium salt; alkaline earth metal salts such as beryllium salt, magnesium salt, calcium salt, strontium salt, and barium salt; aluminum salt; ammonium salt; and phosphonium salt. The other sulfonylimide compounds may be used alone or in combination of two or more. In addition, commercially available products may be used as the other sulfonylimide compounds, or those synthesized by conventionally known methods may be used.

[0033] Examples of the non-imide salt include salts of non-imide anions and cations (lithium ions and the cations exemplified above). [C2] LiPF a (C m F 2m+1 ) 6-a (a:0≦a≦6, m:1≦m≦4)···(2) (hereinafter referred to as "fluorophosphate compound (2)"), a compound represented by general formula (3): [C3] LiBF b (C n F 2n+1 ) 4-b (b:0≦b≦4, n:1≦n≦4)···(3) Examples of the non-imide salt include a compound represented by the formula (hereinafter referred to as "fluoroborate compound (3)"), lithium salts such as lithium hexafluoroarsenate (LiAsF), LiSbF, LiClO, LiSCN, LiAlF, CFSOLi, LiC[(CFSO)], LiN(NO), and LiN[(CN)]; and non-lithium salts (for example, salts in which the lithium (ion) in these lithium salts is substituted with one of the cations exemplified above (e.g., NaBF, NaPF, NaPF(CF)). The non-imide salts may be used alone or in combination of two or more. Furthermore, commercially available non-imide salts may be used, or those obtained by synthesis using a conventionally known method may be used.

[0034] Among the other electrolytes, non-imide salts are preferred from the viewpoints of ionic conductivity, cost, etc., and fluorophosphate compound (2), fluoroborate compound (3) and LiAsF6 are preferred, with fluorophosphate compound (2) being more preferred.

[0035] Examples of the fluorophosphate compound (2) include LiPF, LiPF(CF), LiPF(C,F), LiPF(C,F), LiPF(C,F), etc. Among the fluorophosphate compounds (2), LiPF and LiPF(C,F) are preferred, with LiPF being more preferred.

[0036] Examples of the fluoroboric acid compound (3) include LiBF, LiBF(CF), LiBF(C,F), LiBF(C,F) and LiBF(C,F), etc. Among the fluoroboric acid compounds (3), LiBF and LiBF(CF) are preferred, and LiBF is more preferred.

[0037] These electrolyte salts (sulfonylimide compound (1), other electrolyte salts, etc.) may be present (contained) in the form of ions in the non-aqueous electrolyte solution.

[0038] The electrolyte salt composition may be an electrolyte salt having a simple salt composition of sulfonylimide compound (1), or an electrolyte salt having a mixed salt composition containing sulfonylimide compound (1) and another electrolyte. When an electrolyte salt having a mixed salt composition is used, an electrolyte salt having a mixed salt composition containing sulfonylimide compound (1) and fluorophosphate compound (2) is preferred, and an electrolyte salt having a mixed salt composition containing LiN(FSO2)2 and LiPF6 is more preferred.

[0039] When using an electrolyte salt having a mixed salt composition containing sulfonylimide compound (1) and other electrolytes, the concentration of the other electrolytes in the nonaqueous electrolyte (content, or the total content when two or more types are used in combination) is preferably 0.1 mol / L or more, more preferably 0.2 mol / L or more, even more preferably 0.5 mol / L or more, even more preferably 0.7 mol / L or more, and even more preferably 1 mol / L or more, from the viewpoint of improving battery performance. Furthermore, from the viewpoint of suppressing a decrease in battery performance due to an increase in electrolyte viscosity and self-discharge of the battery, the concentration is preferably 5 mol / L or less, more preferably 3 mol / L or less, even more preferably 2 mol / L or less, and even more preferably 1.5 mol / L or less.

[0040] The total concentration of the electrolyte salts in the nonaqueous electrolyte is preferably 0.8 mol / L or more, more preferably 1 mol / L or more, and even more preferably 1.2 mol / L or more from the viewpoint of improving battery performance, and is preferably 5 mol / L or less, more preferably 3 mol / L or less, and even more preferably 2 mol / L or less from the viewpoint of suppressing a decrease in battery performance due to an increase in the viscosity of the electrolyte.

[0041] From the viewpoint of improving battery performance, it is preferable to increase the concentration of sulfonylimide compound (1). The molar ratio of sulfonylimide compound (1) to other electrolytes (the molar ratio of sulfonylimide compound (1) concentration to other electrolyte concentration) is preferably 1:25 or more, more preferably 1:10 or more, even more preferably 1:8 or more, even more preferably 1:5 or more, even more preferably 1:2 or more, particularly preferably 1:1 or more, and is preferably 25:1 or less, more preferably 10:1 or less, even more preferably 5:1 or less, and even more preferably 2:1 or less.

[0042] (Electrolyte solvent) The electrolyte solvent contains at least one carbonate solvent (hereinafter also referred to as "specific carbonate solvent") selected from the group consisting of chain carbonate solvents and saturated cyclic carbonate solvents. That is, the nonaqueous electrolyte secondary battery according to this embodiment uses, as a constituent material, a nonaqueous electrolyte containing, as an essential component, a chain carbonate solvent and / or a saturated cyclic carbonate solvent together with the sulfonylimide compound (1). In other words, the electrolyte solvent may contain only one or more chain carbonate solvents, only one or more saturated cyclic carbonate solvents, or a mixed carbonate solvent containing a chain carbonate solvent and a saturated cyclic carbonate solvent. Among these, mixed carbonate solvents are preferred, and those containing EMC and EC, which will be described later, are more preferred.

[0043] Examples of chain carbonate (carbonate ester) solvents include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), diphenyl carbonate, and methyl phenyl carbonate. The chain carbonate solvents may be used alone or in combination of two or more. Among the chain carbonate solvents, DMC, EMC, and DEC are preferred, and EMC is more preferred.

[0044] Examples of saturated cyclic carbonate solvents include ethylene carbonate (EC), propylene carbonate (PC), 2,3-dimethylethylene carbonate, 1,2-butylene carbonate, and erythrityl carbonate. The saturated cyclic carbonate solvents may be used alone or in combination of two or more. Among the saturated cyclic carbonate solvents, EC and PC are preferred, and EC is more preferred.

[0045] As described above, the electrolyte solvent may contain the specific carbonate-based solvent, but may also contain other electrolyte solvents (electrolyte solvents other than the specific carbonate-based solvent). The other electrolyte solvents are not particularly limited as long as they can dissolve and disperse the electrolyte salt, and examples thereof include non-aqueous solvents other than the specific carbonate-based solvents, and any solvent generally used in batteries can be used.

[0046] The non-aqueous solvent is preferably a solvent having a high dielectric constant, high solubility for the electrolyte, a boiling point of 60° C. or higher, and a wide electrochemical stability range, and more preferably an organic solvent with a low water content. Examples of such organic solvents include, other than specific carbonate solvents, cyclic carbonate solvents having an unsaturated bond such as methyl vinylene carbonate, ethyl vinylene carbonate, 2-vinyl ethylene carbonate, and phenyl ethylene carbonate; fluorine-containing cyclic carbonate solvents such as fluoroethylene carbonate (FEC), 4,5-difluoroethylene carbonate, and trifluoropropylene carbonate; ether solvents such as ethylene glycol dimethyl ether, ethylene glycol diethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, tetrahydropyran, crown ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,4-dioxane, and 1,3-dioxolane; aromatic carboxylic acid ester solvents such as methyl benzoate and ethyl benzoate; and γ-butyrolactone. Examples of suitable solvents include lactone-based solvents such as γ-valerolactone and δ-valerolactone; phosphate-based solvents such as trimethyl phosphate, ethyl dimethyl phosphate, diethyl methyl phosphate and triethyl phosphate; nitrile-based solvents such as acetonitrile, propionitrile, methoxypropionitrile, glutaronitrile, adiponitrile, 2-methylglutaronitrile, valeronitrile, butyronitrile and isobutyronitrile; sulfur compound-based solvents such as dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, sulfolane, 3-methyl sulfolane and 2,4-dimethyl sulfolane; aromatic nitrile-based solvents such as benzonitrile and tolunitrile; nitromethane, 1,3-dimethyl-2-imidazolidinone, 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone, 3-methyl-2-oxazolidinone, etc.; and chain ester-based solvents such as ethyl acetate, butyl acetate and propyl propionate. These solvents may be used alone or in combination of two or more.

[0047] The electrolyte solvent may be used as a medium such as a polymer or polymer gel used in place of the electrolyte solvent. When a polymer or polymer gel is used in place of the electrolyte solvent, the following methods may be employed. That is, a method in which a solution in which an electrolyte salt is dissolved in an electrolyte solvent is dropped onto a polymer film formed by a conventionally known method to impregnate and support the electrolyte salt and electrolyte solvent; a method in which a polymer and an electrolyte salt are melted and mixed at a temperature equal to or higher than the melting point of the polymer, and then a film is formed, and the film is impregnated with the electrolyte solvent (these are referred to as gel electrolyte); a method in which a nonaqueous electrolyte in which an electrolyte salt is dissolved in an electrolyte solvent in advance is mixed with a polymer, and then the mixture is formed into a film by a casting method or a coating method, and the electrolyte solvent is volatilized; a method in which a polymer and an electrolyte salt are melted at a temperature equal to or higher than the melting point of the polymer, mixed, and molded (true polymer electrolyte); etc.

[0048] Examples of polymers that can be used in place of the electrolyte solvent include polyethylene oxide (PEO), which is a homopolymer or copolymer of epoxy compounds (ethylene oxide, propylene oxide, butylene oxide, allyl glycidyl ether, etc.), polyether polymers such as polypropylene oxide, methacrylic polymers such as polymethyl methacrylate (PMMA), nitrile polymers such as polyacrylonitrile (PAN), fluorine-based polymers such as polyvinylidene fluoride (PVdF) and polyvinylidene fluoride-hexafluoropropylene, and copolymers thereof. These polymers may be used alone or in combination of two or more.

[0049] (additives) The additives are carbon dioxide (CO2), carbon monoxide (CO), bicarbonate ions (HCO3 - ) and carbonate ions (CO3 2- ), at least one selected from the group consisting of (hereinafter, CO and the like are collectively referred to as "carbonic acid component"), an unsaturated cyclic carbonate compound, a compound represented by the general formula (4): [C4] MPO c F d (M: alkali metal element, c: 1≦c≦3, d: 1≦d≦3) (4) (hereinafter referred to as "fluorophosphate compound (4)") and a compound represented by the general formula (5): [5] [-P(=O)(OR 1 )O-] n ···(5) The nonaqueous electrolyte secondary battery according to this embodiment includes at least one phosphorus-atom-containing compound (hereinafter referred to as "phosphorus-atom-containing compound (5)") (hereinafter also referred to as "specific additive"). That is, the nonaqueous electrolyte secondary battery according to this embodiment includes, as a single constituent material, a nonaqueous electrolyte containing, as an essential component, a carbonic acid component such as CO, an unsaturated cyclic carbonate compound, a fluorophosphate compound (4), and a phosphorus-atom-containing compound (5), in addition to a sulfonylimide compound (1) and a specific carbonate solvent. By further using a specific additive in the nonaqueous electrolyte containing the sulfonylimide compound (1) and the specific carbonate solvent, self-discharge of the battery caused by the sulfonylimide compound (1) (particularly LiN(FSO)), is suppressed, and storage characteristics are improved, even when a negative electrode containing graphite with low crystallinity (the "first graphite" described below) is used.

[0050] The specific additive may contain only one or more carbonic acid components such as CO, may contain only one or more unsaturated cyclic carbonate compounds, may contain only one or more fluorophosphate compounds (4), may contain only one or more phosphorus atom-containing compounds (5), or may be a combination (use in combination) of these.

[0051] CO2, CO, HCO3 - and CO3 2- At least one of the following (carbonated components) The use of a carbonate component such as CO2 as an additive means that a predetermined amount or more (e.g., 20 mass ppm or more) of the carbonate component is dissolved in a nonaqueous electrolyte containing sulfonylimide compound (1). In other words, the nonaqueous electrolyte according to this embodiment has a carbonate component dissolved therein as an additive. Note that, when the nonaqueous electrolyte according to this embodiment contains a specific additive other than the carbonate component, the carbonate component does not necessarily have to be dissolved therein.

[0052] In this specification, dissolution of a carbonate component in a non-aqueous electrolyte solution containing sulfonylimide compound (1) means intentionally dissolving a carbonate component in the non-aqueous electrolyte solution, but does not exclude, for example, a carbonate component contained in a raw material for the non-aqueous electrolyte solution, such as an electrolyte solvent, or a carbonate component that is inevitably dissolved in the non-aqueous electrolyte solution during a normal production process for the non-aqueous electrolyte solution or a secondary battery. In other words, the total amount of dissolved carbonate components described below may include not only the intentionally dissolved carbonate component, but also the carbonate component in the raw material and the unavoidably dissolved carbonate component.

[0053] The form of the carbonate component dissolved in the non-aqueous electrolyte is not particularly limited, and may be CO2, CO, HCO3 - and CO3 2- It is sufficient that the compound exists in at least one of the above forms, and it may exist in any one of the above forms or in a plurality of forms.

[0054] From the viewpoint of suppressing self-discharge of the battery, the total dissolved amount of carbonate components in the nonaqueous electrolyte is, for example, 20 mass ppm or more, preferably 50 mass ppm or more, more preferably 100 mass ppm or more, even more preferably 150 mass ppm or more, even more preferably 200 mass ppm or more, still more preferably 250 mass ppm or more, and particularly preferably 500 mass ppm or more, relative to the electrolyte. The upper limit of the total dissolved amount is not particularly limited, but is, for example, equal to or less than the saturated concentration at 25°C. The total dissolved amount can be measured by the method described in the Examples below, for example, gas chromatography.

[0055] In this specification, the total dissolved amount of carbonate components in the non-aqueous electrolyte solution means: In the preparation process of the non-aqueous electrolyte, the total amount of dissolved carbonate components in the electrolyte immediately after preparation, or after an aging period (for example, one week) has elapsed as needed to stabilize the amount of dissolved carbonate components, or This refers to the total amount of dissolved carbonate components in the electrolyte extracted from a secondary battery, for example, in a nitrogen atmosphere, after the battery has been subjected to an aging process in the manufacturing process of the secondary battery. Examples of the aging process include the following processes and the conditions described in the Examples below. (I) After filling and partial charging, the battery is subjected to high-temperature treatment (storage) at 30°C or higher for at least 6 hours for a period of up to 28 days. After degassing and resealing, the battery is checked for initial performance defects through charging and discharging, and then held at 50% charge for at least one week to check for defects due to self-discharge. (II) The same process as (I) except that the high temperature treatment is not performed after the partial charge. (III) The same process as (I) except that degassing is not performed after the high-temperature treatment.

[0056] Examples of the method for dissolving a carbonate component in a non-aqueous electrolyte solution containing the sulfonylimide compound (1) include: (A) a method for dissolving a carbonate component in a non-aqueous electrolyte solution during the preparation of the non-aqueous electrolyte solution; and (B) a method for dissolving a carbonate component in a non-aqueous electrolyte solution during the production of a secondary battery.

[0057] In the (A) nonaqueous electrolyte preparation step, the method of dissolving a carbonate component in a nonaqueous electrolyte is, in other words, a method of using a nonaqueous electrolyte containing a sulfonylimide compound (1) and having a predetermined amount of carbonate component dissolved therein (hereinafter also referred to as a "carbonate-dissolved electrolyte") and injecting the electrolyte into a secondary battery. Examples of methods for dissolving a carbonate component in a nonaqueous electrolyte (dissolving step) include contacting the nonaqueous electrolyte with a gas containing a carbonate component (contacting step), bubbling the nonaqueous electrolyte with a gas containing a carbonate component (bubbling step), stirring the nonaqueous electrolyte in a gas atmosphere containing a carbonate component (stirring step), contacting the nonaqueous electrolyte with a high-pressure gas containing a carbonate component (pressurizing the nonaqueous electrolyte with a gas containing a carbonate component, pressurizing step), and adding a substance that generates a gas containing a carbonate component to the nonaqueous electrolyte (adding step). Examples of substances that generate a gas containing a carbonate component include bicarbonates, carbonates, and dry ice. Furthermore, since a carbonic acid component can be dissolved in an electrolyte solvent generally used for a nonaqueous electrolyte, a nonaqueous electrolyte may be prepared by dissolving the sulfonylimide compound (1) in an electrolyte solvent in which a carbonic acid component has been dissolved in advance. The same method as described above can be used to dissolve the carbonic acid component in the electrolyte solvent. Another method includes a method (substitution step) in which a pre-prepared nonaqueous electrolyte is placed in a sealed container to fill approximately 1 / 10 of its volume, the container is evacuated to a substantially vacuum state, and then the container is filled with the carbonic acid component. This procedure is repeated multiple times to replace the air in the container with the carbonic acid component, and the container is then sealed and stored at a low temperature for several days. The dissolving step may include at least one of the above-described steps, or a combination of multiple steps. The dissolving step preferably includes at least one of a pressurizing step, a liquid contacting step, a bubbling step, and a substituting step. More preferably, the dissolving step includes at least one of a pressurizing step, a liquid contacting step, and a bubbling step. A pressurizing step and a substituting step (or a combination of a pressurizing step and a substituting step) are even more preferred.

[0058] In the method (A), the secondary battery may be assembled in a CO atmosphere or an atmosphere containing CO from the viewpoint of controlling the total amount of dissolved carbonic acid components in the nonaqueous electrolyte to a constant level. Specifically, the step of injecting a nonaqueous electrolyte solution containing dissolved carbonic acid components into the battery and the steps after the injection may be performed in a CO atmosphere or an atmosphere containing CO. After the injection of the electrolyte, the battery may be exposed to a high-pressure CO atmosphere.

[0059] The carbonate-dissolved electrolyte used in the method (A) is obtained by the method for producing a nonaqueous electrolyte according to this embodiment, which includes a dissolving step including at least one of the steps described above for dissolving a predetermined amount of carbonate in a nonaqueous electrolyte containing a sulfonylimide compound (1).

[0060] In the (B) secondary battery manufacturing process, examples of a method for dissolving a carbonate component in a non-aqueous electrolyte include assembling a secondary battery in a CO2 atmosphere and injecting a non-aqueous electrolyte into the battery (specifically, a method in which the inside of a battery exterior sealed on three sides is made into a nearly vacuum state and then filled with CO2, and then non-aqueous electrolyte is injected into the one unsealed side and sealed at normal pressure); and a method in which the air in the battery is replaced with CO2 after non-aqueous electrolyte is injected into the secondary battery. The method for replacing the air in the battery with CO2 can be the same as the method for replacing the air in the container with CO2. Specifically, the air in the exterior can be replaced with CO2 by repeating the operation of creating a nearly vacuum state inside the exterior exterior into which non-aqueous electrolyte has been injected and then filling it with CO2 multiple times.

[0061] The total amount of dissolved carbonic acid components in the non-aqueous electrolyte solution varies depending on the temperature of the non-aqueous electrolyte solution, and therefore it is preferable that the temperature be controlled to a constant value during the process of preparing the non-aqueous electrolyte solution and / or the process of manufacturing the secondary battery.

[0062] <Unsaturated cyclic carbonate compounds> The use of an unsaturated cyclic carbonate compound as an additive refers to the inclusion of a predetermined amount or more of the unsaturated cyclic carbonate compound in a nonaqueous electrolyte solution containing the sulfonylimide compound (1). In other words, the nonaqueous electrolyte solution according to this embodiment contains the unsaturated cyclic carbonate compound as an additive. The unsaturated cyclic carbonate compound may be added to the nonaqueous electrolyte solution or may be added during the preparation process of the nonaqueous electrolyte solution. Note that, when the nonaqueous electrolyte solution contains a specific additive other than the unsaturated cyclic carbonate compound, the nonaqueous electrolyte solution may not contain the compound.

[0063] Examples of unsaturated cyclic carbonate compounds include vinylene carbonate (VC), methyl vinylene carbonate, ethyl vinylene carbonate, 2-vinyl ethylene carbonate, and phenyl ethylene carbonate. Each of the unsaturated cyclic carbonate compounds may be used alone, or two or more of them may be used in combination. Among the unsaturated cyclic carbonate compounds, VC is preferred.

[0064] Fluorophosphate compound (4) The use of fluorophosphate compound (4) as an additive refers to the incorporation of a predetermined amount or more of fluorophosphate compound (4) into a nonaqueous electrolyte solution containing sulfonylimide compound (1). In other words, the nonaqueous electrolyte solution according to this embodiment contains fluorophosphate compound (4) as an additive. Fluorophosphate compound (4) may be added to the nonaqueous electrolyte solution or may be added during the preparation process of the nonaqueous electrolyte solution. When the nonaqueous electrolyte solution contains a specific additive other than fluorophosphate compound (4), the compound (4) may not be included.

[0065] In the general formula (4), examples of the alkali metal element represented by M include lithium, sodium, potassium, rubidium, cesium, etc. Among these, lithium is preferred.

[0066] Examples of the fluorophosphate compound (4) include lithium monofluorophosphate (LiPOF) and lithium difluorophosphate (LiPOF). The fluorophosphate compounds (4) may be used alone or in combination of two or more. Among the fluorophosphate compounds (4), LiPOF and LiPOF (containing at least one selected from the group consisting of LiPOF and LiPOF) are preferred, with LiPOF being preferred.

[0067] 《Phosphorus atom-containing compound (5)》 The use of the phosphorus atom-containing compound (5) as an additive refers to the inclusion of a predetermined amount or more of the phosphorus atom-containing compound (5) in a nonaqueous electrolyte containing the sulfonylimide compound (1). In other words, the nonaqueous electrolyte according to this embodiment contains the phosphorus atom-containing compound (5) as an additive. The phosphorus atom-containing compound (5) may be added to the nonaqueous electrolyte or may be added during the preparation process of the nonaqueous electrolyte. Note that when the nonaqueous electrolyte contains a specific additive other than the phosphorus atom-containing compound (5), the compound (5) may not be included.

[0068] General formula (5):[-P(=O)(OR 1 )O-] n In R 1 R represents an alkyl group having 1 to 6 carbon atoms (which may have a substituent), a fluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), an aryl group (which may have a substituent), a silyl group (which may have a substituent), an alkali metal atom, an onium salt, or a hydrogen atom. 1Among these, preferred are linear alkyl groups having 1 to 6 carbon atoms (which may have a substituent), trifluoroalkyl groups having 1 to 6 carbon atoms (which may have a substituent), trialkylsilyl groups having 1 to 6 carbon atoms (which may have a substituent), and silyl groups formed by bonding an alkyl group having 1 to 6 carbon atoms (which may have a substituent) to two alkyl groups having 1 to 6 carbon atoms (which may have a substituent) that differ in the number of carbon atoms, structure (linear, cyclic, etc.), etc.; linear alkyl groups having 1 to 3 carbon atoms (which may have a substituent), trifluoroalkyl groups having 1 to 3 carbon atoms (which may have a substituent), More preferred are a trialkylsilyl group having 1 to 4 carbon atoms (which may have a substituent), and a silyl group in which an alkyl group having 1 to 4 carbon atoms (which may have a substituent) is bonded to two alkyl groups having 1 to 6 carbon atoms (which may have a substituent) that differ in the number of carbon atoms, structure (chain, cyclic, etc.); an ethyl group, a trifluoroethyl group, a trimethylsilyl group, a triethylsilyl group, a triisopropylsilyl group, a (tertiary (tert-)butyl)dimethylsilyl group, and a (tert-butyl)diphenylsilyl group are even more preferred; a trimethylsilyl group is particularly preferred. In this specification, a trialkylsilyl group having 1 to 6 or 1 to 4 carbon atoms refers to a silyl group in which three alkyl groups having 1 to 6 or 1 to 4 carbon atoms are bonded. In addition, in general formula (5), R 1 may be a trialkoxysilyl group (-Si(-OR)3) in which three alkyl groups (R) having 1 to 6 carbon atoms or 1 to 4 carbon atoms are bonded to a silyl group via an oxygen atom. Examples of trialkoxysilyl groups include a trimethoxysilyl group, a triethoxysilyl group, a triisopropoxysilyl group, a (tert-butoxy)dimethoxysilyl group, and a (tert-butoxy)diphenoxysilyl group. The three alkyl or alkoxy groups may be the same or different. In general formula (5), R 1 are preferably the same group.

[0069] In the general formula (5), n represents an integer of 2 or more (degree of polymerization), for example, n=2 to 200.

[0070] Specific examples of the phosphorus atom-containing compound (5) include ethyl polyphosphate (in general formula (5), R 1 represents an ethyl group), trimethylsilyl polyphosphate (in general formula (5), R 1 represents a trimethylsilyl group (TMS), triethylsilyl polyphosphate (in general formula (5), R 1 represents a triethylsilyl group (TES), poly(triisopropylsilyl)phosphate (in general formula (5), R 1 represents a triisopropylsilyl group (TIPS). ], polyphosphate [(tert-butyl)dimethylsilyl] [in general formula (5), R 1 represents a (tert-butyl)dimethylsilyl group (TBDMS). ], polyphosphate [(tert-butyl)diphenylsilyl] [in general formula (5), R 1 represents a (tert-butyl)diphenylsilyl group (TBDPS), trimethoxysilyl polyphosphate (in general formula (5), R 1 represents a trimethoxysilyl group), triethoxysilyl polyphosphate (in general formula (5), R 1 represents a triethoxysilyl group), polyphosphate (triisopropoxysilyl) (in general formula (5), R 1 represents a triisopropoxysilyl group.], polyphosphate [(tert-butoxy)dimethoxysilyl] [in general formula (5), R 1 represents a (tert-butoxy)dimethoxysilyl group.], polyphosphate [(tert-butoxy)diphenoxysilyl] [in general formula (5), R 1 represents a (tert-butoxy)diphenoxysilyl group. ]. The phosphorus atom-containing compounds (5) may be used alone or in combination of two or more. Among the phosphorus atom-containing compounds (5), trimethylsilyl polyphosphate is preferred.

[0071] Trimethylsilyl polyphosphate is, for example, 31By measuring the presence or absence of peaks indicating bonds between phosphorus atoms and surrounding groups and their abundance ratios (integral ratios of each peak) using P-NMR or the like, structures such as a chain structure represented by the following structural formula (5-1), a cyclic structure represented by the structural formula (5-2), and a branched structure represented by the structural formula (5-3) can be analyzed. 31 The P-NMR measurement conditions include those described in the Examples below.

[0072] [ka]

[0073] [ka]

[0074] [ka]

[0075] In structural formulas (5-1) to (5-3), "TMS" represents a trimethylsilyl group, n represents the degree of polymerization, and Pt, Pm, and Pb represent the peaks of phosphorus atoms. Pt represents the peak of a phosphorus atom at a terminal or side chain, specifically a phosphorus atom having one group in which the hydrogen atom of a hydroxyl group is substituted with another adjacent phosphorus atom (hereinafter also referred to as an "-OP group"). Pm represents the peak of a phosphorus atom with a linear structure, specifically a phosphorus atom having two "-OP groups." Pb represents the peak of a phosphorus atom with a branched structure, specifically a phosphorus atom having three "-OP groups." In this way, the presence or absence of branched structures, their abundance ratio, the degree of polymerization n, etc. can be estimated from the presence or absence of Pt, Pm, and Pb and their integral ratios. For example, if all trimethylsilyl polyphosphates have a chain structure (a structure without a Pb peak) represented by structural formula (5-1), and the integral of the Pt region is set to 1, the degree of polymerization (n) is expressed as 2 × {(integral value of Pt) + (integral value of Pm)} ≒ 2 × {1 + (integral value of Pm)}. Furthermore, if the integral of the Pt region is set to 2, the degree of polymerization (n) is expressed as (integral value of Pt) + (integral value of Pm) ≒ 2 + (integral value of Pm). For example, in the case of trimethylsilyl polyphosphate used in the examples described below, the average degree of polymerization (n) is calculated to be 4.87. It should be noted that if trimethylsilyl polyphosphate contains a cyclic structure represented by structural formula (5-2), the degree of polymerization (n) is estimated to be smaller than the above. Among trimethylsilyl polyphosphates, those with a Pb peak, i.e., those containing a branched structure represented by structural formula (5-3), are preferred from the viewpoints of suppressing battery self-discharge and further improving battery performance. In this case, the integral ratio of Pt, Pm and Pb is preferably Pt:Pm:Pb=1.0:3.0:0.2 to 1.0:9.0:3.0, and more preferably Pt:Pm:Pb=1.0:6.0:0.7 to 1.0:7.0:1.2.

[0076] Among the specific additives, from the viewpoint of suppressing self-discharge of the battery and Ni elution from the positive electrode, a carbonic acid component and a phosphorus atom-containing compound (5) are preferred, a phosphorus atom-containing compound (5) is more preferred, trimethylsilyl polyphosphate, ethyl polyphosphate, (triisopropylsilyl) polyphosphate, and (tert-butyl)dimethylsilyl] polyphosphate are even more preferred, and trimethylsilyl polyphosphate having a branched structure represented by structural formula (5-3) is particularly preferred.

[0077] From the viewpoint of suppressing self-discharge of the battery, the specific additive other than the carbonate component is used in a range of preferably 0.1 mass % to 10 mass % inclusive, more preferably 0.2 mass % to 8 mass % inclusive, even more preferably 0.3 mass % to 5 mass % inclusive, still more preferably 0.3 mass % to 3 mass % inclusive, and even more preferably 0.3 mass % to 1 mass % inclusive, relative to 100 mass % of the total amount of the components contained in the non-aqueous electrolyte.

[0078] Other additives As described above, the non-aqueous electrolyte solution may contain the specific additive, but may also contain other additives (additives other than the specific additive). The other additives are additives intended to improve various properties of the lithium ion secondary battery. The other additives may be added to the non-aqueous electrolyte solution or may be added during the preparation process of the non-aqueous electrolyte solution. Other additives include carboxylic acid anhydrides such as succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, and phenylsuccinic anhydride; sulfur-containing compounds such as ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, busulfan, sulfolane, sulfolene, dimethyl sulfone, tetramethylthiuram monosulfide, and trimethylene glycol sulfate; nitrogen-containing compounds such as 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone, and N-methylsuccinimide; saturated hydrocarbon compounds such as heptane, octane, and cycloheptane; and fluoroethylene carbonate (FEC) and trifluoropropylene. Examples of suitable additives include carbonate compounds such as carbonate, phenylethylene carbonate, and erythritan carbonate; sulfamic acid (amidosulfuric acid, H3NSO3); sulfamate salts (alkali metal salts such as lithium salt, sodium salt, and potassium salt; alkaline earth metal salts such as calcium salt, strontium salt, and barium salt; other metal salts such as manganese salt, copper salt, zinc salt, iron salt, cobalt salt, and nickel salt; ammonium salt; guanidine salt; and fluorooxalate compounds such as lithium salts having an oxalic acid skeleton, such as lithium bis(oxalato)borate (LiBOB), lithium difluorooxalatoborate (LiDFOB), lithium difluorooxalatophosphate (LIDFOP), lithium tetrafluorooxalatophosphate (LITFOP), lithium difluorobis(oxalato)phosphate (LiDFBOP), and lithium tris(oxalato)phosphate. These additives may be used alone or in combination of two or more.The amounts of the other additives added are the same as the amounts of the specific additives mentioned above.

[0079] As described above, the nonaqueous electrolyte according to this embodiment is composed of components such as the sulfonylimide compound (1), a specific carbonate solvent and specific additives, and optionally other electrolyte salts, other electrolyte solvents, other additives, etc. The nonaqueous electrolyte can be prepared, for example, by mixing these components in a predetermined composition (mass) ratio.

[0080] [Positive electrode] The positive electrode includes a positive electrode current collector and a positive electrode mixture layer, and the positive electrode mixture layer is formed on the positive electrode current collector and is usually formed into a sheet shape.

[0081] Examples of metals used for the positive electrode current collector include iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, and platinum. Among these, aluminum is preferred. The shape and dimensions of the positive electrode current collector are not particularly limited.

[0082] The positive electrode mixture layer is formed from a positive electrode mixture (positive electrode composition) that contains a positive electrode active material, a conductive additive, a binder, a solvent for dispersing these components, and the like.

[0083] In the secondary battery according to this embodiment, the positive electrode (positive electrode mixture) is preferably LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM111), LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622), LiNi 0.8 Co 0.1 Mn 0.1 Ternary positive electrode active materials such as O2 (NCM811); LiFePO4, LiFe 0.995 Mn 0.005It is possible to preferably use an iron phosphate-based cathode active material having an olivine structure such as PO4 or the like. These cathode active materials may be used alone or in combination of two or more types.

[0084] Among the ternary cathode active materials, the general formula (6): [Chemical formula 9] Li v Ni x Co y Mn z O 2+w [0.2 ≤ v ≤ 1.2, 0.5 ≤ x ≤ 0.9, 0 < y ≤ 0.3, 0 < z ≤ 0.4, x + y + z = 1, -0.2 ≤ w ≤ 0.2 (v represents the molar ratio of Li, x represents the molar ratio of Ni, y represents the molar ratio of Co, z represents the molar ratio of Mn, and w(2 + w) represents the molar ratio of O.)] ··· (6) [[ID=2]]The high-Ni-containing ternary cathode active material represented by (hereinafter referred to as "high-Ni-containing ternary cathode active material (6)") is preferred.

[0085] In the high-Ni-containing ternary cathode active material (6), the content ratio of Ni (the "x" in the general formula (6)) with respect to the total amount of 100% (100 mol%) on a molar basis of transition metals is 50% or more (0.5 ≤ x), preferably 55% or more (0.55 ≤ x), more preferably 70% or more (0.7 ≤ x). The upper limit of the content ratio is 90% or less (x ≤ 0.9), preferably less than 85% (x < 0.85), more preferably 80% or less (x ≤ 0.8). In addition, the content ratios of components other than Ni in the high-Ni-containing ternary cathode active material (6) (the "v", "y", "z", "w" (2 + w) in the general formula (6)) may be appropriately adjusted within the ranges of the above molar ratios.

[0086] The high-Ni-containing ternary cathode active materials (6) may be used alone or in combination of two or more types. In addition, the high-Ni-containing ternary cathode active material (6) may be a commercially available product or one obtained by synthesis by a conventionally known method. Specific examples of the high-Ni-containing ternary cathode active material (6) include, for example, NCM523, NCM622, NCM811, etc.

[0087] The positive electrode preferably contains at least one of the above-mentioned ternary positive electrode active material and iron phosphate positive electrode active material, but may also contain other positive electrode active materials. The other positive electrode active materials may be any materials capable of absorbing and releasing lithium ions, and may be, for example, positive electrode active materials used in conventionally known secondary batteries (lithium ion secondary batteries).

[0088] Positive electrode active materials used in lithium ion secondary batteries include, for example, lithium cobalt oxide; lithium nickel oxide; lithium manganese oxide; LiNi 1-v-w Co v Al w O2 (0≦v≦1, 0≦w≦1) and other transition metal oxides other than the ternary oxides mentioned above; compounds with an olivine structure such as LiAPO4 (A=Mn, Ni, Co); solid solution materials incorporating multiple transition metals (solid solutions of electrochemically inactive layered Li2MnO3 and electrochemically active layered LiMO2 (M=Co, Ni, or other transition metals)); LiCo x Mn 1-x O2(0≦x≦1);LiNi x Mn 1-x O2 (0≦x≦1); compounds having a fluorinated olivine structure such as Li2APO4F (A=Fe, Mn, Ni, Co); sulfur, etc. can be used. Each of these may be used alone, or two or more types may be used in combination.

[0089] From the viewpoint of improving the output characteristics and electrical characteristics of the secondary battery, the content of the positive electrode active material (total content when multiple positive electrode active materials are included) is preferably 75% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more, relative to 100% by mass of the total amount of components included in the positive electrode composite, and is preferably 99% by mass or less, more preferably 98% by mass or less, and even more preferably 95% by mass or less.

[0090] Conductive additives are used to improve the output of lithium-ion secondary batteries. Conductive carbon is mainly used as the conductive additive. Examples of conductive carbon include carbon black, fibrous carbon, and graphite. Each conductive additive may be used alone, or two or more types may be used in combination. Among conductive additives, carbon black is preferred. Examples of carbon black include ketjen black and acetylene black. From the viewpoint of improving the output characteristics and electrical characteristics of lithium-ion secondary batteries, the content of the conductive additive in the non-volatile matter of the positive electrode mixture is preferably 1 to 20 mass %, more preferably 1.5 to 10 mass %.

[0091] Examples of binders include fluorine-based resins such as polyvinylidene fluoride and polytetrafluoroethylene; synthetic rubbers such as styrene-butadiene rubber and nitrile butadiene rubber; polyamide-based resins such as polyamideimide; polyolefin-based resins such as polyethylene and polypropylene; poly(meth)acrylic resins; polyacrylic acid; and cellulose-based resins such as carboxymethyl cellulose. Each binder may be used alone, or two or more types may be used in combination. Furthermore, the binder may be in a state of being dissolved in a solvent or dispersed in a solvent when used.

[0092] Examples of the solvent include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, tetrahydrofuran, acetonitrile, acetone, ethanol, ethyl acetate, and water. Each of the solvents may be used alone, or two or more of them may be used in combination. The amount of the solvent used is not particularly limited and may be determined appropriately depending on the production method and the materials used.

[0093] The positive electrode mixture may contain other components as needed, such as non-fluorinated polymers such as (meth)acrylic polymers, nitrile polymers, and diene polymers; polymers such as fluorinated polymers such as polytetrafluoroethylene; emulsifiers such as anionic emulsifiers, nonionic emulsifiers, and cationic emulsifiers; dispersants such as polymer dispersants such as styrene-maleic acid copolymers and polyvinylpyrrolidone; thickeners such as carboxymethyl cellulose, hydroxyethyl cellulose, polyvinyl alcohol, polyacrylic acid (salts), and alkali-soluble (meth)acrylic acid-(meth)acrylic acid ester copolymers; preservatives, etc. The content of other components in the non-volatile content of the positive electrode mixture is preferably 0 to 15% by mass, more preferably 0 to 10% by mass.

[0094] The positive electrode mixture can be prepared, for example, by mixing a positive electrode active material, a conductive additive, a binder, a solvent, and other components as necessary, and dispersing the mixture using a bead mill, a ball mill, an agitator mixer, or the like.

[0095] The method for forming the positive electrode (coating method) is not particularly limited, and examples thereof include: (1) a method in which a positive electrode composite is applied to a positive electrode current collector by a conventional coating method (e.g., a doctor blade method, etc.) (and then dried); (2) a method in which a positive electrode current collector is immersed in the positive electrode composite (and then dried); (3) a method in which a sheet formed from the positive electrode composite is bonded to a positive electrode current collector (e.g., bonded via a conductive adhesive) and pressed (and then dried); (4) a method in which a positive electrode composite to which a liquid lubricant has been added is applied or cast onto a positive electrode current collector, formed into a desired shape, and then the liquid lubricant is removed (and then stretched in uniaxial or multiaxial directions); and (5) a method in which a positive electrode composite (or a solid content forming a positive electrode composite layer) is slurried with an electrolyte, transferred in a semi-solid state to a current collector (positive electrode current collector), and used as an electrode (positive electrode) without drying.

[0096] The positive electrode mixture layer may be dried or pressed after being formed or coated (applied), as needed.

[0097] [Negative electrode] The negative electrode includes a negative electrode current collector and a negative electrode mixture layer, and the negative electrode mixture layer is formed on the negative electrode current collector and is usually formed into a sheet shape.

[0098] Examples of metals used for the negative electrode current collector include iron, copper, aluminum, nickel, stainless steel (SUS), titanium, tantalum, gold, and platinum. Among these, copper is preferred. The shape and dimensions of the negative electrode current collector are not particularly limited.

[0099] The negative electrode mixture layer is formed from a negative electrode mixture (negative electrode composition) that contains a negative electrode active material, a conductive additive, a binder, a solvent for dispersing these components, and the like.

[0100] In the nonaqueous electrolyte secondary battery according to this embodiment, the negative electrode active material has a peak area ratio (D / G ratio) of the D band and the G band as analyzed by Raman spectroscopy of greater than 0.7 and / or a half width of the G band as analyzed by Raman spectroscopy of 28 cm -1 and a D / G ratio of 0.7 or less and / or a G band half width of 28 cm -1 The negative electrode includes a negative electrode containing 0 to 10 mass% of the second graphite, with respect to 100 mass% of the total amount of the first graphite and the second graphite. That is, the negative electrode (negative electrode active material) may contain only the "first graphite," or may be a mixed graphite containing the "first graphite" and a "second graphite" whose content is 10 mass% or less. As described above, the nonaqueous electrolyte secondary battery according to this embodiment uses a negative electrode containing the "first graphite" with low crystallinity as an essential component as a single constituent material. By using a negative electrode active material (negative electrode) containing the "first graphite" and a predetermined amount or less of the "second graphite" in combination with a nonaqueous electrolyte containing an intentionally dissolved carbonate component and / or a nonaqueous electrolyte containing a specific additive other than the carbonate component, the self-discharge of the battery caused by the sulfonylimide compound (1) (particularly LiN(FSO2)2) is suppressed, and the storage characteristics are improved.

[0101] Increasing the content of the highly crystalline "second graphite" reduces the self-discharge suppression effect of the battery, but also reduces the cycle capacity retention rate (charge-discharge cycle characteristics). This is thought to be because, when a negative electrode containing the highly crystalline "second graphite" is used, the battery capacity decreases, resulting in a shortened battery life. Furthermore, in a battery using a negative electrode containing the "second graphite," the chargeable capacity is low, resulting in a deep depth of charge for the graphite as a whole, and significant capacity degradation during charge-discharge cycles. In contrast, in the nonaqueous electrolyte secondary battery according to this embodiment, when the negative electrode (negative electrode active material) is a mixed graphite containing the "first graphite" and the "second graphite," the content of the "second graphite" relative to the total amount of the "first graphite" and the "second graphite" (100% by mass) is specified to be 10% by mass or less. This suppresses the decrease in the cycle capacity retention rate and improves the charge-discharge cycle characteristics.

[0102] When the negative electrode active material is mixed graphite, the content of the "second graphite" relative to the total amount (100% by mass) of the "first graphite" and the "second graphite" is 10% by mass or less, preferably 8% by mass or less, and more preferably 5% by mass or less, from the viewpoint of improving battery capacity. The lower limit is 0% by mass or more, and from the viewpoint of suppressing self-discharge of the battery, it is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more.

[0103] The "peak area ratio of the D band and G band analyzed by Raman spectroscopy (D / G ratio)" refers to the peak area ratio of the D band and G band at 1580 cm due to the graphite structure (crystallinity) contained in the carbon material in the Raman spectrum measured using Raman light excited by a laser with a wavelength of 532 nm. -1 The area of ​​the peak intensity I near 1350 cm caused by defects in the graphite structure contained in the carbon material. -1 This refers to the ratio of the area of ​​the peak intensity ID near the center (ID / IG, D band / G band area ratio). Note that these peaks are within ±10 cm -1It does not matter if the D / G ratio is slightly off. The D / G ratio of the "first graphite" is greater than 0.7, and the upper limit is not particularly limited, but is, for example, 2 or less. The "first graphite" having a D / G ratio within the above range refers to graphite with low crystallinity and relatively many disorders and defects in the graphite structure. On the other hand, the D / G ratio of the "second graphite" is 0.7 or less, and the lower limit is not particularly limited, but is, for example, 0.05 or more. The "second graphite" having a D / G ratio within the above range refers to graphite with high crystallinity and relatively few disorders and defects in the graphite structure. Examples of methods for measuring Raman spectra include the methods described in the Examples below.

[0104] The "G-band half-width analyzed by Raman spectroscopy" refers to the G-band half-width at 1580 cm caused by the graphite structure contained in the carbon material in the Raman spectrum measured using Raman light excited by a laser with a wavelength of 532 nm. -1 The G-band half-width is related to the crystallinity of the graphite structure or the amount of disorder and defects. The G-band half-width of "first graphite" is 28 cm -1 The upper limit is not particularly limited, but is, for example, 50 or less. The "first graphite" having a G-band half width within the above range is graphite with low crystallinity and relatively many irregularities and defects in the graphite structure. On the other hand, the G-band half width of the "second graphite" is 28 cm -1 The above effect is achieved if the distance is less than 23cm. -1 The lower limit is not particularly limited, but is, for example, 10 cm -1 The "second graphite" having a G-band half-width within the above range is graphite with high crystallinity and relatively few disturbances and defects in the graphite structure.

[0105] Examples of the "first graphite" include natural graphite such as O-MAC manufactured by Osaka Gas Chemicals Co., Ltd. and SMG manufactured by Hitachi Chemical Co., Ltd. The "first graphite" is a carbon material that is relatively cheaper than the "second graphite." The "first graphite" may be used alone or in combination of two or more types.

[0106] Examples of the "second graphite" include graphite such as MAGE manufactured by Hitachi Chemical Co., Ltd. and SFG15 and SLP50 manufactured by Imerys Co., Ltd. The "second graphite" may be used alone or in combination of two or more types.

[0107] A specific example of mixed graphite is one containing SMG and SFG15 in a composition (mass) ratio of 99-90:1-10.

[0108] As described above, the negative electrode active material may contain "first graphite" and a predetermined amount or less of "second graphite," but may also contain other negative electrode active materials. Examples of other negative electrode active materials that can be used include conventionally known negative electrode active materials used in various batteries (e.g., lithium secondary batteries) and the like, as long as they are capable of absorbing and releasing lithium ions. Examples of other negative electrode active materials that can be used include carbon materials such as mesophase sintered bodies made from coal or petroleum pitch, and non-graphitizable carbon; Si-based negative electrode materials such as Si, Si alloys, and SiO; Sn-based negative electrode materials such as Sn alloys; lithium metal; and lithium alloys such as lithium-aluminum alloys. The other negative electrode active materials may be used alone or in combination of two or more.

[0109] The negative electrode mixture may further contain a conductive additive (conductive substance), a binder, a solvent, etc. The conductive additive, binder, solvent, etc. may be the same components as those described above. The proportions used are also the same as those described above.

[0110] The negative electrode may be manufactured by the same method as the positive electrode.

[0111] (separator) The nonaqueous electrolyte secondary battery according to this embodiment may include a separator. The separator is disposed to separate the positive electrode from the negative electrode. There are no particular limitations on the separator, and any conventionally known separator can be used in the present disclosure. Specific examples of the separator include a porous sheet made of a polymer capable of absorbing and retaining an electrolyte (nonaqueous electrolyte) (e.g., a polyolefin-based microporous separator or a cellulose-based separator), a nonwoven fabric separator, a porous metal body, and the like.

[0112] Examples of the material for the porous sheet include polyethylene, polypropylene, and a laminate having a three-layer structure of polypropylene / polyethylene / polypropylene.

[0113] Examples of materials for the nonwoven fabric separator include cotton, rayon, acetate, nylon, polyester, polypropylene, polyethylene, polyimide, aramid, and glass. Depending on the required mechanical strength, the above-mentioned materials may be used alone or in combination of two or more.

[0114] (battery exterior materials) A battery element including a positive electrode, a negative electrode, and a non-aqueous electrolyte (and a separator) is usually housed in a battery exterior material to protect the battery element from external impacts during battery use, environmental degradation, etc. The material of the battery exterior material is not particularly limited, and any conventionally known exterior material can be used.

[0115] If necessary, the battery exterior may contain expanded metal, an overcurrent prevention element such as a fuse or a PTC element, lead plates, etc. to prevent pressure buildup inside the battery and overcharging and discharging.

[0116] The shape of the battery (lithium ion secondary battery, etc.) is not particularly limited, and any of the conventionally known shapes of batteries (lithium ion secondary batteries, etc.) can be used, such as cylindrical, square, laminated, coin, large, etc. Furthermore, when used as a high-voltage power source (several tens to several hundreds of volts) to be mounted on electric vehicles, hybrid electric vehicles, etc., it can also be made into a battery module consisting of individual batteries connected in series.

[0117] The rated charging voltage of a secondary battery (lithium ion secondary battery, etc.) is not particularly limited, but when the secondary battery has a positive electrode containing the above-mentioned ternary positive electrode active material as a main component, it may be, for example, 3.6 V or higher, preferably 4.0 V or higher, more preferably 4.1 V or higher, and even more preferably 4.2 V or higher. The higher the rated charging voltage, the higher the energy density can be, but from the viewpoint of safety, etc., the rated charging voltage may be, for example, 4.6 V or lower (e.g., 4.5 V or lower).

[0118] <Manufacturing method of non-aqueous electrolyte secondary battery> The nonaqueous electrolyte secondary battery according to this embodiment can be easily manufactured, for example, by stacking a positive electrode and a negative electrode (with a separator interposed therebetween as necessary), placing the resulting laminate in a battery exterior material, injecting a nonaqueous electrolyte into the battery exterior material, and sealing the battery exterior material.

[0119] As described above, the nonaqueous electrolyte secondary battery according to this embodiment, which includes the sulfonylimide compound (1), uses a specific nonaqueous electrolyte containing a specific carbonate-based solvent and a specific additive, in combination with a specific negative electrode containing, as the negative electrode active material, either "first graphite" alone or a mixed graphite containing "first graphite" and a mixed graphite in which the content of "second graphite" is 10% by mass or less relative to 100% by mass of the total of "first graphite" and "second graphite." This allows the nonaqueous electrolyte secondary battery to achieve the effect of improving self-discharge (storage characteristics) by combining the specific negative electrode with the specific additive, and further achieve the synergistic effect of improving charge-discharge cycle characteristics by specifying the content ratio of "second graphite" in the mixed graphite. [Example]

[0120] The present disclosure will be described below based on examples. Note that the present disclosure is not limited to the following examples, and the following examples can be modified or changed based on the spirit of the present disclosure, and such modifications are not excluded from the scope of the present disclosure.

[0121] <Raman spectroscopy of graphite> Using a JASCO NRS-3100 (manufactured by JASCO Corporation), Raman spectroscopy was carried out on various graphites shown in Table 1 under the following conditions. In the obtained Raman spectra (D / G chart, see Figures 1 to 5), -1 The area of ​​the peak intensity I G around 1350 cm is defined as the "G band peak area." -1 The area of ​​the peak intensity ID near 1580 cm was taken as the "D band peak area," and the ratio of these areas was calculated as the "D band and G band peak area ratio (D / G ratio)." -1 The half-width of the peak intensity I G around this point was calculated as the "G-band half-width." The results are shown in Table 1. (Raman spectroscopy measurement conditions) Laser wavelength: 532nm Exposure time: 5 seconds x 4 times ·Center wave number: 2250cm -1 Slit: φ0.2mm -Dimmer: OD1 (laser output 0.7mW) Objective lens: 20x Baseline correction (400cm -1 ~2400cm -1 (linear correction between

[0122] [Table 1]

[0123] <Example 1 Series> (Manufacturing of negative electrodes) (Production Example 1) An aqueous slurry of MAGE (Hitachi Chemical Co., Ltd.): carbon fiber (VGCF, Showa Denko K.K.): carboxymethyl cellulose (CMC, commercially available): styrene butadiene rubber (SBR, commercially available) = 100:2:1:1 (mass ratio, same below) was prepared and coated on one side of copper foil (coating weight 9.8 mg / cm). 2 ) and dried, followed by roll pressing to produce Negative Electrode 1-1.

[0124] (Production Example 2) SFG15 (manufactured by Imerys): A water-based slurry with a composition of VGCF:CMC:SBR=100:2:1:1 was prepared and coated on one side of copper foil (coating weight 9.8 mg / cm 2 ) and dried, followed by roll pressing to produce Negative Electrode 1-2.

[0125] (Production Example 3) SLP50 (Imerys): A water-based slurry with a composition of VGCF:CMC:SBR = 100:2:1:1 was prepared and coated on one side of copper foil (coating weight 9.8 mg / cm). 2 ) and dried, followed by roll pressing to produce Negative Electrode 1-3.

[0126] (Production Example 4) O-MAC (Osaka Gas Chemicals Co., Ltd.): A water-based slurry with a composition of VGCF:CMC:SBR = 100:2:1:1 was prepared and coated on one side of copper foil (coating weight 9.8 mg / cm). 2 ) and dried, followed by roll pressing to produce Negative Electrode 1-4.

[0127] (Production Example 5) A water-based slurry of SMG (manufactured by Hitachi Chemical Co., Ltd.) with a composition of VGCF:CMC:SBR=100:2:1:1 was prepared and applied to one side of copper foil (coating weight 9.8 mg / cm). 2 ) and dried, followed by roll pressing to produce Negative Electrode 1-5.

[0128] (Production Example 6) A water-based slurry of mixed graphite (SMG) and SFG15 (95:5 by mass ratio), VGCF, CMC, and SBR (100:2:1:1) was prepared and applied to one side of copper foil (coating weight 9.8 mg / cm).2 ), dried, and roll-pressed to produce Negative Electrode 1-6.

[0129] (Production Example 7) A water-based slurry of mixed graphite (SMG) and SFG15 (90:10 mass ratio), VGCF, CMC, and SBR (100:2:1:1 composition) was prepared and applied to one side of copper foil (coating weight 9.8 mg / cm). 2 ) and dried, followed by roll pressing to produce Negative Electrode 1-7.

[0130] (Production Example 8) A water-based slurry of mixed graphite (SMG) and SFG15 (85:15 mass ratio), VGCF, CMC, and SBR (100:2:1:1 composition) was prepared and applied to one side of copper foil (coating weight 9.8 mg / cm). 2 ) and dried, followed by roll pressing to produce Negative Electrode 1-8.

[0131] (Production Example 9) O-MAC and SFG15 were mixed in a mass ratio of 85:15 to prepare a water-based slurry with a composition of mixed graphite:VGCF:CMC:SBR=100:2:1:1, and this slurry was applied to one side of copper foil (coating weight 9.8 mg / cm). 2 ) and dried, followed by roll pressing to produce Negative Electrode 1-9.

[0132] [Production of Laminated Batteries (Examples 1-1 to 1-10, Comparative Examples 1-1 to 1-7)] ·Ternary positive electrode (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (NCM111, manufactured by Umicore), acetylene black (Denka, Denka Black), graphite (Nippon Graphite, SP270), and PVdF (Kureha, L#7208) were weighed in a composition (mass) ratio of 100:3:3:3 and dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a slurry. The prepared slurry was coated on one side of aluminum foil (coating weight 19.8 mg / cm). 2 ), dried, and then roll-pressed to produce a positive electrode. The resulting positive electrode was cut into a shape with an effective area of ​​12 cm. 2 Cut to. The negative electrode obtained in each manufacturing example had an effective area of ​​13.44 cm 2 The types of negative electrodes used are shown in Table 2 below. An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Corporation) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) of ethylene carbonate (EC):ethyl methyl carbonate (EMC) = 3:7 (volume ratio) as the electrolyte solvent to a concentration of 0.6 mol / L. The additives shown in Table 2 were added to the solution obtained above to achieve the contents or dissolved amounts shown in Table 2, and the mixture was stirred for one day to prepare a nonaqueous electrolyte (hereinafter also simply referred to as "electrolyte"). A cell was fabricated by ultrasonically welding polarity leads to the cut positive and negative electrodes, placing them face-to-face with a 25 μm polyethylene (PE) separator, and sealing the three sides with a laminate exterior. 700 μL of the electrolyte shown in Table 2 was injected into one of the unsealed sides of the resulting cell, producing a 30 mAh laminate battery (lithium ion battery) as a nonaqueous electrolyte secondary battery. After the electrolyte was injected, the battery was charged at a constant current of 6 mA for 3 hours, one piece was opened, and the battery was vacuum-sealed again to release the gas. After the release, the cell was stored at 25°C for 48 hours, and then charged and discharged under the following conditioning condition 1 to complete the evaluation battery. (Conditioning condition 1) 1st cycle: 3mA, constant current constant voltage charge at 4.2V, terminated at 0.3mA ⇒ Discharge: Discharge at 6mA, terminate at 2.75V. 2nd cycle: 6mA, constant current / constant voltage charge at 4.2V, terminated at 0.6mA ⇒ Discharge: Discharge at 6mA, terminate at 2.75V. 3rd cycle: 6mA, constant current / constant voltage charge at 4.2V, terminated at 0.6mA ⇒ Discharge: Discharge at 30mA, terminate at 2.75V.

[0133] [Evaluation of laminated batteries] [Measurement of self-discharge capacity (OCV) (evaluation of storage characteristics)] The evaluation battery was fully charged by constant current and constant voltage charging at 30 mA and 4.2 V with a cutoff of 0.6 mA. The open circuit voltage (OCV) of the battery after full charge was measured (initial OCV). After measurement, the battery was stored at 60°C for 28 days, and then at 25°C for 4 hours, after which the OCV was measured (OCV after 28 days (storage) at 60°C). Using these measurements, the difference (ΔV) between the initial OCV and the OCV after storage was calculated as self-discharge. The results are shown in Table 2. Note that the higher the OCV after storage, i.e., the smaller the self-discharge (ΔV), the more suppressed the battery's self-discharge (better storage characteristics).

[0134] [Measurement of capacity retention rate at 45°C for 300 cycles (evaluation of charge-discharge cycle characteristics)] The evaluation battery was subjected to a cycle test at 45° C. under the following charge / discharge conditions (cycle conditions) for a total of 300 cycles. The capacity retention rate after 300 cycles was calculated using the following formula (1): [Number 1] Capacity retention rate (%) = (1C capacity at 300th cycle / 1C capacity at 1st cycle) × 100 (1) The results are shown in Table 2. Note that a higher cycle capacity retention rate means better charge-discharge cycle characteristics. (Cycle conditions) Charging: 4.2V, constant current / constant voltage charging at 1C (30mA), stopping at 0.02C (0.6mA), resting for 10 minutes. Discharge: Constant current (CC) discharge at 1C (30mA), terminate at 2.75V, rest for 10 minutes.

[0135] [Table 2]

[0136] [Discussion of Example 1 Series] The D / G ratio (D / G band area ratio) is greater than 0.7 and / or the G band half width is 28 cm -1The batteries (Comparative Examples 1-1 to 1-2) using only relatively inexpensive graphite (O-MAC or SMG, "first graphite") that is larger than the above and has low crystallinity, have a D / G ratio of 0.7 or less and / or a G-band half width of 28 cm -1 Compared to batteries (Comparative Examples 1-3 to 1-5) that used only the following highly crystalline, relatively expensive graphite (MAGE, SFG15, or SLP50, "second graphite"), it was confirmed that the OCV was lower and the self-discharge (ΔV) increased after storage at 60°C for 28 days, indicating a large self-discharge from a fully charged state. It was confirmed that the batteries (Examples 1-1 to 1-6) using a specific electrolyte solution containing VC or LiPO2F2 as a specific additive or a specific electrolyte solution containing dissolved CO2 in combination with "first graphite" had a higher OCV after storage and a smaller self-discharge than the batteries (Comparative Examples 1-3 to 1-5) using a conventional electrolyte solution not containing a specific additive in combination with "second graphite." Therefore, it ... dissolved CO2 in combination with "first graphite" had a higher OCV after storage and a smaller self-discharge than the batteries (Comparative Examples 1-3 to 1-5) using a specific electrolyte solution containing no specific additive in combination with "second graphite." -1 It was confirmed that the self-discharge problem, which was a problem with batteries that used a combination of the relatively inexpensive "first graphite," which is larger and has lower crystallinity than the conventional electrolyte containing LiFSI, can be suppressed. The detailed mechanism behind why the self-discharge of a battery containing graphite with a larger D / G ratio and / or G-band half-width is greater is unknown, but one possible factor is the selective decomposition of LiFSI on the graphite surface corresponding to the D-band. The reason why adding and dissolving VC, LiPO2F2, or CO2 into a conventional electrolyte containing LiFSI improves the self-discharge of the battery is thought to be because the additives are decomposed first at the negative electrode, suppressing the decomposition of FSI to form a coating. The batteries (Comparative Examples 1-6 to 1-7) using mixed graphite containing "first graphite" and "second graphite" and containing 15% by mass of "second graphite" relative to 100% by mass of the total amount of "first graphite" and "second graphite" showed a lower capacity retention rate at 45°C for 300 cycles when used in combination with an electrolyte containing VC or an electrolyte containing dissolved CO2, although self-discharge was suppressed compared to the batteries (Examples 1-7 to 1-10) using mixed graphite containing 5% by mass or 10% by mass of "second graphite". The reason for this is that the D / G ratio is less than 0.7 and / or the G-band half width is 28 cm -1 The highly crystalline "secondary graphite" described below has a low lithium absorption capacity, so as the content of "secondary graphite" in the mixed graphite increases, the absorption capacity of the negative electrode decreases, which is thought to be the reason for the decrease in cycle capacity retention rate. On the other hand, it was confirmed that by specifying the content of "secondary graphite" in mixed graphite to be 10 mass% or less, batteries using electrolytes containing added VC or LiPO2F2 or electrolytes containing dissolved CO2 not only have reduced self-discharge but also have reduced decline in cycle capacity retention, i.e., both the storage characteristics and charge / discharge cycle characteristics of the battery are improved.

[0137] <Example 2 Series> (Manufacturing of negative electrodes) The composition is the same as that of negative electrodes 1-1 to 1-9, but the coating weight is 10.8 mg / cm 2 Negative electrodes 2-1 to 2-9 were produced in the same manner as in Production Examples 1 to 9, except for the above change.

[0138] [Production of Laminated Batteries (Examples 2-1 to 2-6, Comparative Examples 2-1 to 2-4)] The positive electrode active material is LiNi manufactured by Beijing Dongben Co., Ltd. 0.8 Co 0.1 Mn 0.1 A positive electrode was produced in the same manner as in Example 1 series, except that O2 (NCM811) was used. A non-aqueous electrolyte solution was prepared in the same manner as in Example 1 series, except that the additives were changed to the types and their contents or dissolved amounts shown in Table 3. Using the obtained positive and negative electrodes, a 30 mAh laminate battery was manufactured in the same manner as in Example 1 series, and was charged and discharged under the above-mentioned conditioning condition 1 to complete the evaluation battery. The types of negative electrode and electrolyte used are shown in Table 3 below.

[0139] [Evaluation of laminated batteries] The test battery was evaluated in the same manner as in the Example 1 series.

[0140] [Table 3]

[0141] [Discussion of Example 2 Series] Positive electrode: LiNi 0.8 Co 0.1 Mn 0.1 Even when changed to O2, it was confirmed that the batteries (each example) using a negative electrode containing the "first graphite" in combination with an electrolyte solution containing VC or LiPO2F2 or an electrolyte solution containing dissolved CO2 had a high OCV after storage and could suppress self-discharge. Furthermore, with regard to the mixed graphite containing the "first graphite" and the "second graphite," as in the Example 1 series, in a battery using a negative electrode containing mixed graphite in which the "second graphite" was 15% by mass relative to 100% by mass of the total amount of the "first graphite" and the "second graphite," self-discharge was suppressed, but a decrease in the capacity retention rate at 45°C for 300 cycles was confirmed.

[0142] <Example 3 Series> (Manufacturing of negative electrodes) The composition is the same as that of negative electrodes 1-1 to 1-9, but the coating weight is 8.8 mg / cm 2 Negative electrodes 2-1 to 2-9 were produced in the same manner as in Production Examples 1 to 9, except for the above change.

[0143] [Production of Laminated Batteries (Examples 3-1 to 3-3, Comparative Examples 3-1 to 3-4)] The positive electrode active material was changed to commercially available LiFePO4, and LiFePO4, acetylene black (HS-100), and PVdF (Kureha, L#7208) were weighed in a composition (mass) ratio of 100:9:6 and dispersed in NMP to prepare a slurry. The prepared slurry was coated on one side of aluminum foil (coating weight 20.20 mg / cm). 2 ) and dried, followed by roll pressing to produce a positive electrode. A non-aqueous electrolyte solution was prepared in the same manner as in Example 1 series, except that the additives were changed to the types and their contents or dissolved amounts shown in Table 4. Using the obtained positive and negative electrodes, a 25 mAh laminate battery was manufactured in the same manner as in Example 1. The types of negative electrode and electrolyte used are shown in Table 4 below. After the electrolyte was injected, the battery was charged at a constant current of 5 mA for 3 hours, one piece was opened, and the battery was vacuum-sealed again to release the gas. After the release, the battery was stored at 25°C for 48 hours, and then charged and discharged under the following conditioning condition 2 to complete the evaluation battery. (Conditioning condition 2) 1st cycle: Charging: 2.5mA, constant current constant voltage charging at 3.6V, termination at 0.25mA ⇒ Discharge: Discharge at 5mA, terminate at 2.0V. 2nd cycle: Charging: 2.5mA, constant current constant voltage charging at 3.6V, termination at 0.5mA ⇒ Discharge: Discharge at 5mA, terminate at 2.0V. 3rd cycle: Charging: 2.5mA, constant current constant voltage charging at 3.6V, termination at 0.5mA ⇒ Discharge: Discharge at 25mA, terminate at 2.0V.

[0144] [Evaluation of laminated batteries] The evaluation battery was charged and discharged under the following conditions (25° C.), and the initial capacity was confirmed. (Conditions) Charging: 3.6V, 25mA constant current constant voltage charging, 0.5mA termination ⇒ Discharge: 2.5mA terminates at 2.0V.

[0145] [Measurement of self-discharge capacity (OCV) (evaluation of storage characteristics)] The OCV was measured in the same manner as in Example 1 series, except that the batteries for which the initial capacity had been measured were charged at a constant current of 25 mA and a constant voltage of 3.6 V with a cut-off of 0.5 mA to reach a fully charged state. Furthermore, after 28 days (storage) at 60°C, the batteries for which the OCV had been measured were discharged at a constant current of 2.5 mA with a cut-off of 2.0 V, and the remaining capacity after 28 days of storage at 60°C was measured. The remaining capacity thus obtained and the initial capacity were used to calculate the formula (2): [Number 2] Remaining rate (%) = remaining capacity / initial capacity × 100 (2) The survival rate was calculated by the above method, and the results are shown in Table 4. A high survival rate means a low self-discharge capacity, which indicates that self-discharge is suppressed.

[0146] [Table 4]

[0147] [Discussion of Example 3 Series] Even when the positive electrode was changed to LiFePO4, it was confirmed that the batteries (each example) using a negative electrode containing the "first graphite" in combination with an electrolyte solution to which VC or LiPO2F2 was added or an electrolyte solution to which CO2 was dissolved had a high OCV after storage and could suppress self-discharge. Because the discharge voltage of the LiFePO4 positive electrode is flat, the relationship between OCV and self-discharge (ΔV) may not be clear. Therefore, when the remaining rate during storage was checked, it was confirmed that the remaining rate was high in each example, and self-discharge was suppressed. Furthermore, with regard to the mixed graphite containing the "first graphite" and the "second graphite," as in the Example 1 series, in a battery using a negative electrode containing mixed graphite in which the "second graphite" was 15% by mass relative to 100% by mass of the total amount of the "first graphite" and the "second graphite," self-discharge was suppressed, but a decrease in the capacity retention rate at 45°C for 300 cycles was confirmed.

[0148] <Example 4 Series> [Synthesis and Analysis of Additives] ( 31 P-NMR analysis) Trimethylsilyl polyphosphate (hereinafter referred to as "PPSE-1"), a reagent manufactured by Sigma-Aldrich, was used. 31 When analyzed by P-NMR, two peaks were confirmed, one appearing at a chemical shift of -28 ppm to -33 ppm (Pt) and the other appearing at a chemical shift of -35 ppm to -41 ppm (Pm), as shown in Figure 6. The integral ratio of the two peaks was Pt:Pm = 1.00:1.43. On the other hand, the peak appearing at -41 ppm to -45 ppm (Pb) was not confirmed. 31P-NMR measurements were performed using a JNM-ECA500 manufactured by JEOL (Japan Electronics Corporation) with a double sample tube as the sample tube, and the chemical shift was determined with the phosphorus peak of H3PO4 added to one of the tubes as 0 ppm.

[0149] (Synthesis of trimethylsilyl polyphosphate 1) 1.553 g of diphosphorus pentoxide was dispersed in 10 mL of methylene chloride solvent, and 1.710 g of hexamethylenedisiloxane was gradually added dropwise while stirring. The mixture was then stirred at room temperature for about a day. The solvent was then distilled off to synthesize polytrimethylsilyl phosphate (hereinafter also referred to as "PPSE-2"). ( 31 P-NMR analysis) The synthesized trimethylsilyl polyphosphate (PPSE-2) 31 Analysis by P-NMR in the same manner as above confirmed three peaks: a peak (Pt) appearing at chemical shifts of -28 ppm to -33 ppm, a peak (Pm) appearing at chemical shifts of -35 ppm to -41 ppm, and a peak (Pb) appearing at chemical shifts of -41 ppm to -45 ppm, as shown in Figure 7. The integral ratio of the three peaks was Pt:Pm:Pb = 1.00:6.73:1.00. From these analysis results, PPSE-2 is presumed to be polytrimethylsilyl phosphate containing a large amount of branched structures (branched structures represented by the above structural formula (5-3)).

[0150] (Synthesis of trimethylsilyl polyphosphate 2) 1.553 g of diphosphorus pentoxide was dispersed in 10 mL of toluene as a solvent, and 1.710 g of hexamethylenedisiloxane was gradually added dropwise while stirring. The mixture was then stirred at room temperature for about a day. The solvent was then distilled off to synthesize polytrimethylsilyl phosphate (hereinafter also referred to as "PPSE-3"). ( 31 P-NMR analysis) The synthesized trimethylsilyl polyphosphate (PPSE-3) 31Analysis by P-NMR in the same manner as above confirmed three peaks: a peak (Pt) appearing at chemical shifts of -28 ppm to -33 ppm, a peak (Pm) appearing at chemical shifts of -35 ppm to -41 ppm, and a peak (Pb) appearing at chemical shifts of -41 ppm to -45 ppm, as shown in Figure 8. The integral ratio of the three peaks was Pt:Pm:Pb = 1.00:6.51:0.81. From these analysis results, PPSE-3 is presumed to be polytrimethylsilyl phosphate containing a large amount of branched structures (branched structures represented by the above structural formula (5-3)).

[0151] (Synthesis of ethyl polyphosphate) 10 g of diphosphorus pentoxide, 10 g of chloroform as a solvent, and 20 g of diethyl ether were placed in a test tube and stirred at 900 rpm at 35°C for 3 days. The solvent was then removed using an evaporator, and the residue was vacuum dried for 24 hours to synthesize ethyl polyphosphate (hereinafter also referred to as "PPE"). ( 31 P-NMR analysis) The synthesized polyethyl phosphate (PPE) 31 Analysis by P-NMR in the same manner as above confirmed three peaks: a peak (Pt) appearing at chemical shifts of -12 ppm to -15 ppm, a peak (Pm) appearing at -25 ppm to -31 ppm, and a peak (Pb) appearing at -39 ppm to -46 ppm. The integral ratio of the three peaks was Pt:Pm:Pb = 1.00:3.59:0.42. From these analysis results, it is estimated that PPE is polyethyl phosphate containing a large amount of branched structures (branched structures represented by the above structural formula (5-3) in which TMS is an ethyl group).

[0152] (Synthesis of triisopropylsilyl polyphosphate) 0.53 g of indium(III) bromide was dissolved in 30 mL of tetrahydrofuran, followed by the addition of 4.75 g of triisopropylsilane. The reaction was allowed to proceed with stirring at room temperature for one day. 30 mL of hexane was added to the reaction solution, which was then allowed to stand, resulting in separation into two layers. The separated upper layer was concentrated under reduced pressure to yield 4.01 g of hexaisopropyldisiloxane, a colorless liquid. Next, 1.55 g of phosphorus pentoxide and 1.70 g of the hexaisopropyldisiloxane obtained above were added to 10 mL of dichloromethane, and the reaction was allowed to proceed with stirring at 35°C for three days. The reaction solution was filtered and concentrated under reduced pressure to synthesize 2.10 g of poly(triisopropylsilyl)phosphate (hereinafter referred to as "PPSE(TIPS)") as a viscous liquid.

[0153] (Synthesis of (tert-butyl)dimethylsilyl polyphosphate) Poly(tert-butyl)dimethylsilyl]phosphate (hereinafter also referred to as "PPSE(TBDMS)") was synthesized by the same procedure as in the synthesis of PPSE(TIPS), except that the triisopropylsilane used in the synthesis of PPSE(TIPS) was changed to tert-butyldimethylsilane.

[0154] [Production of Laminated Batteries (Examples 4-1 to 4-9, Comparative Examples 4-1 to 4-2)] The positive electrode active material is LiNi manufactured by Beijing Dongben Co., Ltd. 0.8 Co 0.1 Mn 0.1 Changed to O2 (NCM811), coating weight 15.7 mg / cm 2 A positive electrode was produced in the same manner as in Example 1 series, except that the above was changed. The negative electrode 2-1 or 2-5 prepared in Example 2 series was used. A non-aqueous electrolyte solution was prepared in the same manner as in Example 1 series, except that the additives were changed to the types and contents or dissolved amounts shown in Tables 5 and 6. Using the obtained positive and negative electrodes, a 30 mAh laminate battery was manufactured in the same manner as in Example 1. The types of negative electrode and electrolyte used are shown in Tables 5 and 6 below. After the electrolyte was injected, the battery was vacuum sealed and charged at a constant current of 3 mA at 25°C for 3 hours. It was then left at room temperature for 2 days, and one piece of the laminate exterior was cleaved and vacuum sealed again to allow for degassing. After degassing, the battery was charged and discharged under the following conditioning condition 3 to complete the evaluation battery. (Conditioning condition 3) 1st cycle: 3mA, constant current constant voltage charge at 4.2V, terminated at 0.3mA ⇒ Discharge: Discharge at 6mA, terminate at 2.75V. 2nd cycle: 15mA, constant current / constant voltage charge at 4.2V, terminated at 0.6mA ⇒ Discharge: Discharge at 6mA, terminate at 2.75V. 3rd cycle: 15mA, constant current / constant voltage charge at 4.2V, terminated at 0.6mA ⇒ Discharge: Discharge at 30mA, terminate at 2.75V. 4th cycle: 15mA, constant current / constant voltage charge at 4.2V, terminated at 0.6mA ⇒ Discharge: Discharge at 60mA, terminate at 2.75V.

[0155] [Evaluation of laminated batteries] [Measurement of self-discharge capacity (OCV) (evaluation of storage characteristics)] The conditioned batteries were charged to a full charge state using a charge-discharge tester at a constant current of 30 mA (1C), 4.2 V, and a 0.6 mA cutoff. The fully charged batteries were stored at 60°C for 28 days or at 80°C for 14 days, and then the OCV was measured in the same manner as in Example 1, except that they were left at 25°C for at least 6 hours. The results are shown in Tables 5 and 6.

[0156] [Detected amount of transition metals] After the OCV measurement, the battery was discharged, then disassembled. The removed PE separator and negative electrode were washed with 50 mL of EMC and dried. The negative electrode composite (negative electrode active material) was then peeled from the current collector (copper foil) and immersed in 1 g of nitric acid for 24 hours to dissolve. The PE separator was then also immersed in the same nitric acid for 24 hours. The resulting nitric acid solution was filtered and diluted with ultrapure water to prepare a measurement sample. The measurement sample was analyzed using an ICP atomic emission spectrometer (Shimadzu Corporation) to determine the amount of nickel (Ni) in the measurement sample (per battery (cell)). The results are shown in Tables 5 and 6.

[0157] [Table 5]

[0158] [Table 6]

[0159] [Discussion of Example 4 Series] In the electrolyte without additives, the D / G ratio is 0.7 or less (or the G-band half-width is 28 cm -1 Compared with a battery (Comparative Example 4-2) using a negative electrode containing the "second graphite" (see below), the D / G ratio is greater than 0.7 (or the G-band half width is 28 cm -1 Although the battery (Comparative Example 4-1) using a negative electrode containing "first graphite" (larger than 100%) experienced increased self-discharge (ΔV), it was confirmed that adding various additives shown in Tables 5 and 6 to the electrolyte (each Example) suppressed not only self-discharge during storage at 60°C for 28 days, but also self-discharge during storage at 80°C for 14 days. It was also confirmed that the amount of Ni eluted due to side reactions was reduced. Specifically, this is as follows. It was confirmed that adding polytrimethylsilyl phosphate (PPSE-1) to the electrolyte suppressed self-discharge during 28 days of storage at 60°C and 14 days of storage at 80°C, even when using a negative electrode containing "first graphite," compared to when using a negative electrode containing "second graphite." Furthermore, the addition of PPSE-1 reduced the amount of Ni detected by ICP analysis, confirming that Ni elution from the positive electrode was suppressed. The use of polytrimethylsilyl phosphate (PPSE-2, PPSE-3), which contains a high proportion of branched structures, also showed a similar trend to that of the commercially available polytrimethylsilyl phosphate (PPSE-1). In other words, the addition of PPSE-2 or PPSE-3 to the electrolyte suppressed self-discharge and reduced the amount of Ni detected during storage at 60°C for 28 days and at 80°C for 14 days. While the reasons for this are unclear, PPSE-2 and PPSE-3, which contain a peak (Pb) indicating a branched structure, detected less Ni than PPSE-1, which does not exhibit a Pb peak. This further suppressed Ni elution from the positive electrode. This is thought to be the reason for the improvement in self-discharge. The use of polyethyl phosphate (PPE), which contains a large amount of branched structures, showed a similar tendency to that of polytrimethylsilyl phosphate (PPSE-1). In other words, it was confirmed that adding PPE to the electrolyte suppressed self-discharge during storage at 60°C for 28 days and at 80°C for 14 days, and reduced the amount of Ni detected. The use of poly(triisopropylsilyl)phosphate [PPSE(TIPS)] or poly(tert-butyl)dimethylsilyl]phosphate (PPSE(TBDMS)) also showed a similar trend to that of poly(trimethylsilyl)phosphate (PPSE-1). In other words, it was confirmed that adding PPSE(TIPS) or PPSE(TBDMS) to the electrolyte suppressed self-discharge during storage at 60°C for 28 days and at 80°C for 14 days, and reduced the amount of Ni detected.

Claims

1. A lithium ion secondary battery comprising a positive electrode, a nonaqueous electrolyte solution containing an electrolyte salt and an electrolyte solvent, and a negative electrode containing a negative electrode active material, The electrolyte salt contains a sulfonylimide compound represented by general formula (1), the electrolyte solvent includes at least one carbonate solvent selected from the group consisting of chain carbonate solvents and saturated cyclic carbonate solvents, the nonaqueous electrolyte solution further contains an unsaturated cyclic carbonate compound, a negative electrode active material comprising a first graphite having a peak area ratio (D / G ratio) of a D band to a G band analyzed by Raman spectroscopy of greater than 0.7, and a second graphite having a D / G ratio of 0.7 or less in an amount of 0% by mass or more and 10% by mass or less, relative to 100% by mass of the total amount of the first graphite and the second graphite. LiN (RSO ２ ) (FSO ２ ) (R represents a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or a fluoroalkyl group having 1 to 6 carbon atoms) ... (1)

2. A lithium ion secondary battery comprising a positive electrode, a nonaqueous electrolyte solution containing an electrolyte salt and an electrolyte solvent, and a negative electrode containing a negative electrode active material, The electrolyte salt contains a sulfonylimide compound represented by general formula (1), the electrolyte solvent includes at least one carbonate solvent selected from the group consisting of chain carbonate solvents and saturated cyclic carbonate solvents, the nonaqueous electrolyte solution further contains an unsaturated cyclic carbonate compound, The negative electrode active material has a G band half width of 28 cm as analyzed by Raman spectroscopy. －１ and a G band half width of 28 cm －１ A lithium ion secondary battery comprising a second graphite having the following content in an amount of 0% by mass or more and 10% by mass or less, relative to 100% by mass of the total amount of the first graphite and the second graphite: LiN (RSO ２ ) (FSO ２ ) (R represents a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or a fluoroalkyl group having 1 to 6 carbon atoms) ... (1)

3. The sulfonylimide compound represented by the general formula (1) is LiN(FSO ２ ) ２ 3. The lithium ion secondary battery according to claim 1, further comprising:

4. 3. The lithium ion secondary battery according to claim 1, wherein the unsaturated cyclic carbonate compound includes vinylene carbonate.

5. The electrolyte salt is a compound represented by general formula (2), a compound represented by general formula (3), and LiAsF ６ 3. The lithium ion secondary battery according to claim 1, further comprising at least one selected from the group consisting of: LiPF ａ (C ｍ F ２ｍ＋１ ) ６－ａ (a:0≦a≦6、m:1≦m≦4)・・・ (2) LiBF ｂ (C ｎ F ２ｎ+１ ) ４－ｂ (b:0≦b≦4、n:1≦n≦4)・・・(3)

6. A lithium ion secondary battery as described in claim 1 or 2, characterized in that the content of the unsaturated cyclic carbonate compound is 0.1 mass% or more and 10 mass% or less, relative to 100 mass% of the total amount of components contained in the non-aqueous electrolyte.

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